This invention relates generally to medical apparatus for improving venous blood flow from the lower leg. More particularly, the present invention relates to medical apparatus for stimulating the calf muscle pump function.
It is generally assumed that the deep venous system carries 90% of the blood from the lower limb. Transition from rest to normal rhythmic exercise such as walking, is accompanied by dramatic changes in the pressure and the flow in the veins of the lower limb. When a subject moves from a supine to a standing position, the foot venous pressure rises from 15 to around 115 mm Hg because of the hydrostatic pressure. Fifteen to 20% of the total blood volume may pool in the lower extremities and 10% of plasma volume is lost to tissues after 20 to 30 minutes of passive standing. During this time the hydrostatic pressure within the blood vessels of the lower extremities increases leading to an increased transcapillary filtration into the interstitial space. Concomitantly, the re-absorption of interstitial fluid is reduced, resulting in an increased extravascular fluid volume and edema. The longer the blood stagnates in the lower limb the higher the incidence of secondary conditions such as venous insufficiency (varicose vein), blood clot formation in the lower leg (known as deep venous thrombosis or DVT). The clot forming in the leg will be eventually released into the circulating blood and may cause a life threatening condition known as pulmonary embolism (PE).
Any condition that predisposes a person to a stationary position without the opportunity for activation of the physiologic muscle pump will increase that person's chance for the development of venous insufficiency and subsequent DVT, edema and PE. The followings are some of the examples that predispose a person to the development of DVT and PE:                1 Occupations and work posture that may require a person to sit or stand for a long period of time (i.e.: computer programmers, pilots).        2 Airplane passengers during long distance flight in the economy section (this is known as Coach Syndrome or Economy Class Syndrome).        3 Astronauts during weightlessness (zero gravity), which increase the pooling of the blood to the lower limb due to the negative pressure.        4 Patients during prolonged surgery and during anesthesia as well as during recovery.        5 Elderly due to inactivity, chronic disease and wheel chair confinement, and chronic vascular disorders.        6 Finally those with muscle paralysis (i.e.: spinal cord injured, stroke patients and those with multiple sclerosis), which are unable to contract the lower limb muscles due to paralysis.        
In these situations where voluntary calf muscle pump function is not attainable (situational space limitation for movement, i.e.: airplane passengers or occupations), as well as conditions where voluntary muscle activation is not possible due to paralysis (i.e.: spinal cord injury, stroke) another means of activation is required.
Elastic stockings provide a modest benefit in augmenting superficial venous flow velocity. However, they are not effective in improving deep veins flow and have no effect on venous sinuses (where, the clot forms). Such stockings require meticulous care and must be changed routinely.
Intermittent calf compression has been extensively investigated for the prevention of postoperative venous thrombosis. Pressure is applied to the calf by intermittent inflation of a cuff or boot. The external pressure is applied to the calves over the peripheral veins (40 torrs was most often used), followed by a longer period of deflation and then an interval to allow refilling of the veins before recycling every 1–2 minutes., The external pneumatic devices are not portable due to their large body and need an external electrical outlet. Therefore, they are more useful in the operating room and in recovery where patients have limited ambulation. Many of the pneumatic devices in current use are uncomfortable because they produce excessive sweating beneath the plastic sleeves. They cannot be worn while the patient is ambulatory. They are bulky, require a connection to air compressor, and are associated with compartment syndrome.
The action of the calf muscle pump has an important effect in reducing the venous pressure. The normal functioning of the calf muscle pump (also called venous pump of the calf) is defined as the ability to keep the venous outflow from the lower leg equal to the arterial inflow during exercise, without undue dilation of the veins of the lower leg. The muscular pumping mechanism has important functional connotations: it drastically lowers the venous and capillary pressures, reduces the blood volume contained within the veins of the leg. The veins also act as a reservoir that releases stored blood during muscular contractions; momentarily accelerating the return of venous blood from the leg to the central circulations, therefore increasing exercise capability. The muscle pump also prevents the development of edema in the lower extremities by promoting lymph flow in an upright posture.
Functional electrical stimulation (FES) has been used to induce purposeful movements in the paralyzed muscles in the person with spinal cord injury and strokes patients. The basic idea behind FES is to use electrical current at the level appropriate for an individual to induce function in the paralyzed skeletal muscles. Most of the studies have used FES to induce function in the paralyzed muscle of people with spinal cord injury or stroke to cause contraction for the purpose of exercise.
It should be noted that about 90% of the circulating blood is carried through venous system, this system basically acts as reservoirs to release stored blood in its sinusoidal deep veins during muscle contractions, momentarily accelerating the return of venous blood, preventing blood stasis and also preventing edema by promoting lymph flow. Contraction of the calf muscle powerfully compresses the veins, with one-way valves which prevent back flow, and propels the blood to the heart via the venous system. During relaxation, the pressure in the veins drops sharply and refilling results from small capillaries. This mechanism increases pre-load on the heart via the Frank-Starling mechanism. The Frank-Starling mechanism states that an increase in venous return results in a greater enddiastolic volume and within a few beats blood flow out of the heart will equal flow into the heart. The skeletal muscle pump has been referred to as the peripheral venous heart. Therefore activation of the these muscles by FES specially in those who are confined to wheelchair (i.e.: people with paralysis and elderly) will actually improve the circulation of the blood and make more blood available for every day activities, specially wheel chair propulsion and eventually leads to a better quality of life.
Pneumatic venous foot pumps applied to the foot are systems designed to stimulate the venous foot pump artificially by flattening of the plantar arch. The device has been shown to maintain venous circulation as effectively as does normal walking. These systems are also not practical due to power requirements and limitation of movement during their use.
Studies have shown that the soleus muscle and its veins act as a peripheral pump, filling during relaxation and emptying during contraction. Functional neuromuscular electrical stimulation (FES) of the calf musculature duplicates the effects of this pumping mechanism during ambulation and effectively empties the venous blood and improve the blood flow. When analyzing the characteristics of blood flow following FES application in comparison with venous foot pump, it has been shown that venous foot pump caused a steady rise in the velocity of blood in the vein which slowly returned to baseline over a period of two to three seconds. Calf stimulation, however, produced nearly an instantaneous rise in the velocity of the blood flow in the veins, which then fell to zero. The fall to zero of the blood flow in the lower legs of subjects after calf stimulation suggested that it was quickly emptying the vessels on which it was acting and no more flow resulted. The net zero flow could be due to refilling from distal and proximal veins. This fall to zero of blood flow was not seen when the venous foot pump was utilized suggesting that blood is being pushed past the orifices of the sinuses allowing blood to remain stagnant within the sinuses. Analyzing the velocity spectra, the venous foot pumps caused laminar flow within the veins while the calf stimulation caused turbulent. Therefore, calf muscle stimulation is more effective than the venous foot pump because it provided greater peak velocity after stimulation and the appearance of complete purging of the blood from the veins of the lower extremity to the heart.